Do you supplement oyster shells? How long have they been laying? If thy are new sometimes it just happens. If thy are older give them calcium via oyster shells. I just mix some in food for a little bit and they snap right out of it. Hope this helps.

Would raw milk provide the same calcium, or is there a reason to go out and buy oyster shell for them?

The reason I ask is because there is a dairy farm not too far from me, and I'm able to get raw milk from them when it is too old to sell. They can only sell milk for one or two days after they get it from the cow, so if they get more milk than they can sell in that time span, I get their extra for free. My chickens get raw milk several times a week as a result.

But I do have a couple of young pullets who have only been laying about two months now.